Fuel cells fed with a gaseous fuel for generating electricity are electrochemical devices. The gaseous fuel includes, for example, hydrogen and/or methanol and with oxygen or air. These electrochemical devices are known and have been developed over the last few decades. There is a large amount of literature on the different and peculiar technical aspects of these primary generators, such as, for example, the permionic cell separator, the catalytic electrodes, the cell structure, and the current collectors and distributors, for example.
Most commonly, the electrolyte is a polymeric film or membrane of an ion exchange resin, and typically a cation exchange resin often referred to as the proton exchange membrane (PEM). On the opposite faces of the proton exchange membrane are microporous catalytic electrodes permeable to the gaseous reagents, i.e., the fuel gas on one face and the oxygen or oxygen containing gas on the other face.
Cations or most typically protons (H+) that are generated by ionization of hydrogen upon shedding electrons to the positive electrode at sites (triple phase points) at which are mutually in contact with the catalytic material of the electrode. This is in continuity with an electrical path with the negative electrode of the cell through the load circuit. The solid polymer electrolyte and the fuel, such as hydrogen, for example, is fed to the positive electrode of the cell. It migrates under the effect of the electric field through the proton exchange membrane and eventually reaches similar catalytic sites on the negative electrode of the cell disposed on the opposite phase of the membrane. Here the proton H+ combines with oxygen to form water while absorbing electrons from the negative electrode.
It is well known that a fundamental requisite to achieve an acceptable rate (dynamics) of the electrodic anodic and cathodic (half-cell reactions) processes that take place on the opposite faces of the ion exchange membrane forming a solid polymer electrolyte of the cell is the high number of active sites available to support the respective half-cell reaction, in other words, their density per unit area of the cell. For this purpose, the catalytic electrodes are made of metallic structures permeable to gases and have a relatively high specific surface, besides being made of a chemically stable and catalytic metal.
Platinum and often platinum black, either supported or not on high specific surface particles of a chemically inert and electrically conductive material, such as carbon (carbon black), for example, is one of the catalytic electrode materials most often used, though other noble metals such as iridium, palladium, rhodium, ruthenium and alloys thereof may also be used.
Even the techniques used for forming the catalytic electrodes on the opposite faces of the ion exchange membrane must ensure an adequate porosity and permeability of the electrode structure by the respective liquid reagent and/or gas (fuel, oxygen or oxygen containing gas, mixture, water) in order to favor an effective mass transfer between the sites capable of supporting the electrodic reaction (half-cell reaction). In particular, the catalytic electrode forming the negative electrode of the fuel cell must also provide for an unhindered and immediate out flow of any excess water that may be formed as a product of the half-cell reaction.
To meet these basic requisites there have been numerous proposals and techniques for incorporating and/or adhering particles of catalytic electrode materials directly on the opposite faces of the membrane. This is done while ensuring an adequate porosity and permeability of the bonded electrode structure by the gaseous reagents and products, for example, by using as an adhesion binder polytetrafluoroethylene in the form submicrometric fibrous particles.
Another approach is to form suitable electrode structures by microetching. That is, forming electrodes in the form of thin metal films made microporous by a microetching step to be bonded or pressed on the opposite faces of the ion exchange membrane. U.S. Pat. No. 6,136,412 discloses a membrane-electrode assembly in which the electrode in contact with the membrane is formed by a non-structured porous electrically conducting element on which a catalytic metal (Pt) is deposited. The prior art documents cited in this patent provide a vast collection of publications representative of the state of the art of fuel cells.
In all instances, the requisites of microporosity, permeability and catalytic properties of the metal forming the electrode of the cell require the use of appropriate current collectors. These current collectors are capable of establishing electrical contact points that are densely and uniformly distributed over the rear surface. This is on the entire area of the electrode to provide for an adequate current conducting body that is easily connectable to the load circuit of the fuel cell.
In case of preformed catalytic electrodes in the form of thin microporous metal films, the current collectors may even fulfill the other fundamental function of providing for a mechanical support of the microporous catalytic electrode that typically is intrinsically fragile and difficult to handle.
Such an electrochemical device of electrical generation has the advantage of being free of corrosive liquid electrolytes, of toxic compounds and/or pollutants. It also has a very high power/weight ratio, and an autonomy of operation that may be adapted to need (reservoir of usable fuel). The electrochemical device is also simple to form. This makes it particularly suited to power portable instruments and devices, and more particularly, systems that may be entirely integrated monolithically on silicon. These systems include, for example, mobile radios, monitoring instruments, portable computers, signaling devices, radio beacons and the likes. Another important area of application is related to gas sensors that are integrated together with associated monitoring, testing and signaling circuitry formed on silicon.
With such a growing demand for fuel cells having a reduced size for powering portable electronic instruments and apparatuses, the possibility of forming a fuel cell of a very small or minuscule size directly on silicon becomes very relevant. Such a fuel cell could even be integrated (at least a half-cell) on the same chip on which the functional circuitry to be powered by the micro fuel cell is integrated. Even in the area of biochemical process research, micro fuel cells integrated on silicon may be very useful.